From 99f78b8bca55a16c7229ded5fc7f418b3f8af36d Mon Sep 17 00:00:00 2001
From: prenastro <prenastro@gmail.com>
Date: Wed, 8 Jul 2026 02:21:53 -0700
Subject: [PATCH] Completed s30 Trees-1

---
 btfrominorderandpreorder.py | 37 +++++++++++++++++++++++++++++++++++++
 validbinarysearchtree.py    | 20 ++++++++++++++++++++
 2 files changed, 57 insertions(+)
 create mode 100644 btfrominorderandpreorder.py
 create mode 100644 validbinarysearchtree.py

diff --git a/btfrominorderandpreorder.py b/btfrominorderandpreorder.py
new file mode 100644
index 00000000..8f14ff2a
--- /dev/null
+++ b/btfrominorderandpreorder.py
@@ -0,0 +1,37 @@
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def buildTree(self, preorder, inorder):
+
+        inorder_map = {}
+        for i, num in enumerate(inorder):
+            inorder_map[num] = i
+
+        preorder_index = 0
+
+        def dfs(left, right):
+            nonlocal preorder_index
+
+            if left > right:
+                return None
+
+            root_val = preorder[preorder_index]
+            preorder_index += 1
+
+            root = TreeNode(root_val)
+
+            mid = inorder_map[root_val]
+
+            root.left = dfs(left, mid - 1)
+            root.right = dfs(mid + 1, right)
+
+            return root
+
+        return dfs(0, len(inorder) - 1)
+
+    # TC - O(n) because we use hashmap and recursion
+    # SC - O(n) because of hashmap
\ No newline at end of file
diff --git a/validbinarysearchtree.py b/validbinarysearchtree.py
new file mode 100644
index 00000000..25a2e8a4
--- /dev/null
+++ b/validbinarysearchtree.py
@@ -0,0 +1,20 @@
+class Solution:
+    def isValidBST(self, root):
+
+        def dfs(node, low, high):
+            if not node:
+                return True
+
+            # Current node must be within the allowed range
+            if not (low < node.val < high):
+                return False
+
+            # Left subtree: values must be < node.val
+            # Right subtree: values must be > node.val
+            return (dfs(node.left, low, node.val) and
+                    dfs(node.right, node.val, high))
+
+        return dfs(root, float("-inf"), float("inf"))
+
+    # TC - O(n)
+    # SC - O(h) where h is height of tree, O(logn) for balanced binary tree and O(n) for skewed tree.
\ No newline at end of file